An Early Paleozoic collisional event along the northern margin of the Central Tianshan Block: Constraints from geochemistry and geochronology of granitic rocks

• The adakitic granodiorite of 496 Ma records the Paleo-Tianshan Ocean’s subduction.
• The gneissic granite dated at 426 Ma exhibits features of post-orogenic granite.
• The Paleo-Tianshan Ocean closed before 426 Ma.
• The Carboniferous granodiorite and monzodiorite are VAG of the North Tianshan Ocean.

The Tianshan orogen was formed by multiple-stage accretionary events. Ophiolites exposed in the northern margin of the Central Tianshan Block (CTB) indicate the existence of an Early Paleozoic ocean between the Tuha terrane and the CTB, i.e., the Paleo-Tianshan Ocean. However, the closing history of this ocean is still controversial. Granitoids exposed extensively in the CTB can place a good constraint for this collisional process. In this study, geochemical and geochronological analyses are carried out on four granitic plutons from the CTB. One mylonitic granodiorite intruding the CTB basement has a zircon U–Pb age of ca. 496 Ma and shows geochemical affinity to adakite. Its formation can be attributed to the subduction of the Paleo-Tianshan Ocean. The ∼426 Ma old gneissic granite that intrudes the Gangou ophiolitic zone presents geochemical features of A-type granite, and gives an upper limit for the closure time of the Paleo-Tianshan Ocean. The metamorphic age of 430.5 ± 4.6 Ma of the zircons from the mylonitic granodiorite possibly represents the event caused by the collision. Two other granitic samples, a granodiorite and a monzodiorite yield ∼330 Ma and ∼319 Ma in age and display the chemical characters of I-type granites. That indicates a magmatic arc setting mostly related to the subduction of another ocean, called the North Tianshan Ocean.

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